1. Field of the Invention
The invention in general relates to steam turbines, and particularly to an arrangement for testing the steam admission valves thereof.
2. Description of the Prior Art
In the operation of a steam turbine-generator power plant, a steam source produces steam which is provided to a turbine system consisting of one or more turbines. The regulation of the steam through the tubine system is governed by the positioning of a plurality of steam admission valves and as the steam expands through the turbine, or turbines, work is extracted and utilized by an electrical generator for producing electricity which is supplied to a power system network through a circuit breaker arrangement, when closed.
In a typical arrangement, the steam admission valves include a plurality of throttle valves, also known as stop valves, and a plurality of governor valves, also known as control or steam chest valves. The throttle valves are used for controlling the steam flow to the turbine during the period the turbine is being brought up to speed and are additionally used to shut off the flow of steam to the turbine in the event of overspeeding beyond a predetermined value. The governor valves, when in control, provide the function of precisely regulating the speed and load of the turbine by controlling the steam flow.
In the operation of the power plant the throttle valves must be exercised periodically to detect possible valve stem sticking and provisions are made to test close these valves from some test station or operator's panel. In addition, it is often desirable to test individual ones of the governor valves.
Since the lift-versus-flow characteristics of the steam admission valves are non-linear, most advanced control systems generally provide some sort of dynamic valve characteristic and flow compensation arrangement. The present invention relates to an improved valve test system which permits the control response of the valves to remain linear during testing and provides for a stable load level with only minimal reliance on load feedback circuits normally provided with such control systems.